The present invention relates to the field of solid state capacitors. The invention particularly relates to capacitors of the type in which a powder-formed valve action metal forms a highly porous anode body portion of a capacitor, an electrically insulating dielectric layer is formed though the porous structure of the anode body, and a conducting cathode layer is formed on the dielectric layer and which is then electrically connected to a cathode terminal, the anode body being electrically connected to an anode terminal.
U.S. patent specification No. 5,357,399 (Salisbury) describes a method for simultaneously manufacturing multiple such capacitors from a porous tantalum layer sintered to a tantalum substrate. The layer is machined to form anode body portions of each capacitor. After processing a top plate (substrate lid) is bonded to the processed anode body top ends. The plate forms a lid which, after machining of the substrate/anode body/plate sandwich, becomes the cathode terminal of each capacitor. PCT application GB 99/03566 concerns a modified version of the Salisbury method in which the volumetric efficiency of the capacitors produced is optimized by removing the need for a substrate lid as the cathode terminal of each capacitor, thereby increasing the specific capacitive volume.
The foregoing methods permit the manufacture of very small but highly volume efficient capacitors. However the continued pressure of electronic circuit board design towards miniaturization of components and ease of assembly of such boards maintains a continued need for capacitors of improved volumetric efficiency and reduced component windows (or footprint) on the circuit board.
The present invention seeks to provide improved capacitors and improved methods of manufacturing such capacitors.
According to one aspect of the present invention there is provided a method of manufacturing a solid state capacitor comprising: providing an electrically conducting substrate; forming a plurality of upstanding porous electrically conducting anode bodies on a surface of the substrate, each body electrically connected to the substrate; forming an electrically insulating layer on the exposed surface area provided by the porous bodies; forming a conducting layer on the insulating layer; dividing the substrate into capacitor units, each comprising a portion of substrate provided with a porous capacitive body, and for each unit: providing a cathode terminal in electrical contact with the conducting layer on the capacitive body, providing an anode terminal in electrical contact with the substrate portion, characterized in that the cathode terminal is formed on a surface of the capacitive body distal to the substrate portion and the anode terminal is formed adjacent and substantially co-planar with the cathode terminal, an electrically conducting wick providing electrical contact between the substrate portion and the anode terminal, so that the capacitors have anode and cathode terminals on a common face.
By forming a capacitor with anode and cathode connections on a common face the footprint of the capacitor is minimized, whilst facilitating connection with a circuit board.
The electrically conducting wick may be formed by a process in which a plurality of upstanding electrically conducting wick bodies are formed on the surface of the substrate alongside the anode bodies, the substrate division producing capacitor units comprising a portion of substrate provided with both a porous capacitive body and a wick body, and wherein the anode terminal is formed on a surface of the wick body distal to the substrate portion.
In a preferred embodiment, the anode bodies are formed by configuration of a pre-form layer of porous conducting material applied to the surface of the substrate. Conveniently, the wick body is a porous conducting body, which may be formed by configuration of the pre-form layer.
By xe2x80x9cconfigurationxe2x80x9d the reader is intended to understand any shaping or forming process which can form the required bodies. Typical examples are cutting and machining, for example by saws or cutting wheels. However it may be that the worker may wish to employ laser cutting, water cutting, etching or other known methods to form the body shapes.
The wick bodies may be allowed to be provided with insulating and conducting layers along with the anode bodies. In this case an electrical connection through the insulating layer is provided by subsequent removal of the applied layers. This removal may be by machining, cutting, grinding, etching or the like, so long as the underlying conducting wick material is exposed.
In one embodiment of the invention, the dividing of the substrate preferably involves machining or cutting through a plane which passes through the wick bodies, thereby to expose un-coated wick material with which an anode terminal contact may be made. In this way no separate cutting process or machining is necessary to remove the insulating layers; it is included in the substrate division process.
In another embodiment the removal of insulating and conducting layers is carried out on a face of each wick distal to the substrate thereby to expose uncoated wick material with which an anode terminal contact may be made. This has the advantage of exposing a surface which is adjacent and co-planar the anode body top face, simplifying contact with the terminals.
A conductive material bridge may electrically connect the anode terminal and the exposed un-coated wick material. Typically the bridge material is applied as a conducting paste (e.g. silver paste) which sets to from a solid coating. To enhance the contact a carbon layer may first be applied.
In another aspect of the invention there is provided a state capacitor comprising a substrate portion and a capacitive body, which body comprises a porous anode body electrically connected to the substrate portion, an electrically insulating layer formed on the anode body surface area, and a conducting layer formed on the insulating layer, a surface of which capacitive body distal to the substrate portion is provided with a cathode terminal, characterized in that an anode terminal is provided adjacent and substantially co-planar with the cathode terminal, an electrically conducting wick providing electrical contact between the substrate portion and the anode terminal, thereby providing a capacitor having anode and cathode terminals on a common face.
In certain embodiments, in each capacitor, there are a plurality of anode terminals adjacent and substantially co-planar with the cathode terminal, each anode terminal electrically connected to the substrate by an associated wick. The wicks may each be formed form the same porous conducting material as the anode body.
According to another aspect of the invention there is provided a method of manufacturing multiple solid state capacitors comprising a method as hereinbefore described wherein a plurality of anode and wick bodies are formed on the substrate, and the substrate is divided to provide a plurality of individual capacitor units.
As the terminals are coplanar, the capacitor may stand on a flat surface with the cathode terminal and anode terminal contacting the flat surface. This makes the capacitor very well adapted for placement on and attachment to a circuit board.
The pre-form may be applied to the substrate by laying a green, unsintered mixture of valve action metal powder and binder/lubricant on the substrate. The green mixture may then be sintered to fuse the powder into a solid highly porous pre-form, the binder/lubricant being burnt off during sintering.
The pre-form layer maybe machined to form the anode bodies and the wick bodies. Typically longitudinal and lateral grinding cuts may be employed in order to produce an array of rectilinear anode and wick bodies on the substrate, separated by xe2x80x9cstreetsxe2x80x9d corresponding to the path of the grinding cut. Naturally more complex shapes can be produced by conventional machining techniques, as required.
The processing is facilitated if both wick and anode bodies are coated with the insulating layer and the conducting cathode layer. An alternative would be to mask the wick bodies in order to prevent coating of the anode bodies from, but this would be a rather difficult and more complicated process.
The insulating layer may be a dielectric layer of an oxide of the valve action material, applied for example by conventional anodization techniques in order to build up gradually an oxide of the required thickness and integrity. In on example, in which the valve action layer is tantalum, a layer of tantalum pentoxide is built up on the bodies.
The conducting layer may be applied by dipping of the anode and wick bodies into a precursor solution of, for example manganese nitrate solution. The layer of manganese nitrate formed on the bodies may be heated to oxidize the nitrate to manganese dioxide. Repeated dipping steps may be necessary in order to build-up the optimum cathode layer.
Building-up of the conducting or xe2x80x9ccathodexe2x80x9d layer completes the formation of the anode body into a capacitive body.
In the case where both the anode bodies and wick bodies are subject to the application of an insulating layer and a cathode layer it is necessary to isolate electrically the cathode layer material on the anode bodies from that on the wick bodies, in order to prevent a short circuit in the final capacitors. This may involve removal of all cathode layer material bridging the anode and wick bodies. Typically this may be achieved by a grinding cut through the conducting layer, and inevitably through the insulating layer also. In this case a replacement electrically insulating layer may be formed on any exposed surfaces revealed by cathode layer removal. This process is known as reformation. Again this may be conducted by a re-anodization process.
As well as isolating the conducting layers of the respective anode bodies and wick bodies one from another, it is necessary to remove conducting layer and insulating layer material from those parts of the wick bodies which are to contact or form the anode terminals, so that an electrical connection to the valve-action substrate material may be made. Removal of the layers may be by machining, for example grinding. In one example grinding cuts are made along a top surface of each wick body, thereby exposing valve action material. The top surface may then be subjected to a termination process to form the anode terminal. Typically this involves application of a first layer of conducting carbon paste which is then cured. Next a second layer of conducting silver paste is applied, and cured. Finally a solder-facilitating tri-alloy layer, or the like, may be applied to enable a good soldered contact to be made. A similar termination process is also carried out on a top surface of the capacitive body, in which carbon and silver layers are formed on the conducting cathode layer of the top surface, optional followed by application of a tri-alloy layer. These conducting layers provide a terminal for electrical connection, by for example soldering, to an electrical or electronic circuit.
In the un-divided substrate, the spaces between the anode bodies and the cathode bodies may be filled with an insulating material, for example a liquid plastics resin which solidifies to form a protective encapsulation of the bodies. Naturally the resin should leave the upper surface of the capacitive and wick bodies exposed, by masking if necessary. Other wise removal of the resin layer back to expose these faces is required.
The next step which must be carried out is separation of the or each capacitor unit from the bulk substrate. This may be achieved by machining by for example a grinding cut. If necessary a rigid backing support may be provided for the substrate to as to provide the necessary structural rigidity to permit cutting without damaging the capacitors.
In another aspect of the invention the dividing comprises cutting along a plane or path which intersects with one or more wick bodies, thereby to cut through or remove conducting layer material and insulating layer material applied to the wick, and to expose a cut surface of uncoated wick body. Preferably the wick bodies may be arranged on the substrate in rows, and the dividing comprises cutting along one or more of the rows.
The cutting plane preferably intersects with a wick body surface region distal from the associated anode body of the capacitor unit to be divided.
The cutting is preferably carried out through a plane or planes perpendicular or substantially perpendicular to the plane of the substrate. The cutting may comprise grinding, but could also include water cutting or other cutting methods.
The terminal may be provided on the expose cut surface of the wick body by a termination process comprising liquid coating of that surface by a conducting paste, and allowing the coating to solidify. The termination processes may further comprise electroplating the solidified coating to form a layer of metallic material on the respective body or bodies.
Preferably, before dividing, the substrate is coated with a protective insulating material which infiltrates in between the anode and wick bodies, and wherein the dividing process comprises cutting along the protective material, thereby to leave a sidewall of protective material around each anode and cathode body of each cathode portion, the wall being absent in the said side regions of the anode bodies which intersect with the cut.
The protective material may be a resin material which is infiltrated as a liquid and subsequently allowed to set.
A termination layer of metal plate may be applied, for example by electrodeposition. Typically a layer of nickel and tin/lead or gold is applied. This provides a solder compatible surface for electrical connection.
Following is a description by way of example only and with reference to the accompanying drawings of methods of putting the present invention into effect.